Classical sonic logging employs the acquisition and analysis of data with a simple monopole source. For this type of source, physics limits shear speed determination to speeds faster than the acoustic velocity of the borehole fluid. A dipole source excites the borehole flexural mode, providing a means to determine shear speed without this limitation. Propagation models and waveforms from computer Propagation models and waveforms from computer simulations of hard and soft formations with both monopole and dipole sources are presented. These simulations are then compared to laboratory scale model data. Real-world borehole data acquired with both monopole and dipole sources, along with the instrumentation employed to acquire the data, are described. In particular, a combined monopole and dipole logging instrument with a high fidelity monopole and dipole receiver array is discussed. A processing algorithm for extracting compressional, shear and Stoneley speeds from monopole and dipole full waveform data is presented. Log samples are presented for hard, soft and presented. Log samples are presented for hard, soft and extremely soft formations from both monopole and dipole sources. Application of log data to evaluation of rock mechanical properties, such as Poisson's ratio, fracture evaluation, and correlation to compressional and shear seismic data, is also discussed.
Introduction
Since the 1950's, borehole sonic logs have been made with sonic logging tools which incorporate transmitters and receivers which exhibit monopole radiation characteristics. These instruments were first used to detect sonic first arrivals for determination of formation compressional velocities. As sonic logging technology evolved, observing and recording the full wavetrain permitted detection of the later arriving waves associated with shear propagation. From these data, compressional and shear propagation velocities were determined and applied to estimate formation properties, such as rock lithology, fluid content, stiffness, compressibility and other mechanical properties, as well as assisting in the interpretation of surface and borehole seismic data. However, for monopole-based technology, borehole physics limits shear determination to velocities faster than the physics limits shear determination to velocities faster than the acoustic velocity of the borehole fluid. Although these "hard rock" formations are quite common, many important reservoirs fall into the category of "soft rock" or a mixture of both. In these soft, slow, poorly consolidated formations, the shear waves are primarily directed away from the bore hole and direct measurement of the formation shear velocity is not possible. possible. Dipole transmitters and receivers, on the other hand, remove this fluid velocity barrier. In particular, dipole sources produce a flexural mode in the borehole from which the formation shear velocity may be determined regardless of fluid velocity. In the following sections, the borehole physics of monopole and dipole logging tools will be discussed with presentation of computer simulations and comparison with presentation of computer simulations and comparison with laboratory scale model data. Following that discussion, the technology incorporated in a commercial logging instrument combining both monopole and dipole transmitters and receivers will be presented. Waveform processing techniques, example logs from both the hard and soft rock environments and data application examples conclude the discussion.
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